Particles of many materials to be ground deform practically inelastically under heavy loads, such as compression, shear and impact stresses occurring in size reduction machinery. This applies to
materials which have already a low yielding strength at normal temperature, such as many polymers, especially thermoplastic polymers (polytetrafluoroethylene, polyethylene, polypropylene, polyamide, etc.), organic chemicals, and plastic metals (e.g. copper),
materials which, although behaving largely elastically at normal temperature and low stresses, heat up under stress and react increasingly inelastically, such as polystyrene, polymethylmethacrylate,
materials which become plastifiable under high pressures, such as alkalihalides.
Materials are classified as inelastic if their force-deformation (stress-strain) diagram shows no coincidence of the loading and unloading curve. The first one takes a flatter course than the unload curve. The area circumscribed by both curves is a measure of the inelastic deformation. Contrary to that, the load and unload curves coincide if the material is elastic. The inelasticity appears in two forms to be differentiated: viscous flow and plastic deformation. The predominance of either of the two in the material behavior depends on the molecular structure of the substance and, with a given substance, on the temperature, and on velocity, and intensity of the loading. At increasing temperature or decreasing rate of load application the inelastic behavior intensifies.
There is a general tendency that with decreasing particle size the capability of particles to behave inelastically, in other words to deform inelastically, increases. This applies specifically to particle sizes below 1 mm. Therefore, in pulverization even particles of those materials which are normally considered as elastic or only weak inelastic materials, practically behave inelastic.
Inelastic material behavior considerably impedes the comminution of many substances, in particular tough ones. Therefore, it is difficult to grind many of the materials of the above mentioned groups in a particle size range below 1 mm.
The prior art suggested the following possibilities to overcome those difficulties:
reducing the inelasticity by subjecting the material to high deformation rates and/or cooling the material to enhance the development of brittle fractures (embrittlement of the particles), i.e. effect a predominantly elastic deformation of the bodies up to rupture,
subjecting the material to pronounced shearing stress, such as can be obtained at edges and knives (cutting of the particles).
High deformation rates are achieved with impact mills of any kind and air jet mills. These may be operated at high rates of air or gas flow, a fact which facilitates cooling of the material to be ground. Impact mills therefore, are especially well suited for pulverizing inelastic materials. In cases of extreme inelasticity the embrittlement must be effected by means of liquid gas, e.g. liquid nitrogen. The grinding is done in low-temperature milling plants which may comprise mills of any kind, e.g. impact mills, disc mills, vibration mills, and the like.
A pronounced shearing stress at edges and knives is achieved with cut-mills, cutter-granulators, disc mills, and also with impact mills having profiled grinding tools. Cut-mills and cutter-granulators are particularly suitable for producing particles of a size above 1 mm, whereas disc mills and impact mills having profiled grinding tools are used for grinding material down to particle sizes of 200 .mu.m but not less than 100 .mu.m.
The power requirement for fine-grinding inelastic substances or substances which behave inelastically when being subjected to comminution is quite considerable and lies between 50 and 1500 kWh/t, depending on the material and desired degree of fineness. The throughput rate of any given mill is noticeably lowered when higher degrees of fineness have to be produced. With plastics, for instance, this reduction may be 80% if the maximum particle size of the finished material is to be decreased from 800 .mu.m to 200 .mu.m. If such degrees of fineness are desired, the throughputs of mills of ordinary capacity often are no more than 10 to 40 kg/h for very inelastic materials.
It is almost impossible to produce particles of less than 50 .mu.m by grinding if the material is extremely inelastic, such as polyethylene, polytetrafluoroethylene, polypropylene, and copper. Powders of this degree of fineness are obtained by precipitation from solutions or by the spraying of melts.